Intracellular trafficking of organelles and molecules, which is vitally important to the function of neuronal processes and nerve terminals, involves a variety of neuronal proteins that couple motor protein complexes with intracellular cargoes and structures. HAPl, a neural protein that binds to the Huntington's disease protein huntingtin, appears to play roles in intracellular transport and other neuronal functions. First, our studies show that it interacts with dynactin p150 and kinesin light chain, which are involved in microtubule dependent transport. Second, HAP1 is associated with a number of intracellular organelles or structures, especially a unique cytoplasmic inclusion of unknown function. Recently, we also observed that HAP1 is involved in neurite outgrowth and is colocalized with huntingtin in presynaptic terminals. We hypothesize that the interactions of HAP1 with various proteins confer its multiple and diverse functions, which may include the development of neurites and synaptic plasticity. The abnormal interaction of HAP1 with mutant huntingtin may affect some of HAP1's neuronal functions and contribute to the neuropathology of HD. Three aims are proposed to test the above hypothesis. (I) We will identify the regions in HAP1 responsible for binding to KLC or dynactin p150 to examine whether neurite outgrowth is affected by inhibiting the interaction between HAP1 and KLC or dynactin p150. (2) We will identify the nature of the unique cytoplasmic inclusion to understand how HAP1 is associated with this subcellular structure. We will investigate how HAP1 is targeted to synaptic vesicles and whether it is involved in synaptic vesicle recycling. (3) We will examine whether mutant huntingtin affects HAP1's protein interactions and its related functions. These studies aim to advance our understanding of neuronal function of HAP1 and huntingtin. They will also help us to understand the mechanism of the specific neuropathology in Huntington's disease.